1. Field of the Invention
The present invention relates to a device for determining the position of the leading or training edge of a label that is on a continuous sheet, or web, of labels. The method and devices described herein relate to any operation where it's necessary or beneficial to detect either the simple presence or the exact position of a pressure-sensitive label. Examples of such operations can occur during the manufacture of the labels, when the labels are being applied to objects by a machine, when the labels on a roll need to be counted, and so on.
2. Description of the Related Art
Pressure-sensitive adhesive-backed labels are often supplied in a continuous web. The web consists of a backing sheet (or carrier web, or backing—but hereafter in this disclosure called the liner) that is made in a way that allows labels attached to it to be readily removed without damage, and then permanently re-applied onto something else. The liner can be made from a material that has been pretreated on the label side to be slippery, thus allowing easy removal of the labels. Or the liner can be made from a material to which the labels naturally do not permanently adhere. On that liner is a continuous row of labels. So the web comprises the liner, together with all the labels that are attached to it. The labels that are removably attached to these long, continuous liners may be one or the other of two kinds: die-cut labels or butt-cut labels.
Die-cut labels have a typical gap of 3.175 mm (0.125 inch) between adjacent labels, and are narrower than the liner on which they're manufactured, so there is exposed liner all around every die-cut label.
Butt-cut labels, however, are attached to the liner edge-to-edge with no space between adjacent labels, and may extend all the way across the width of the liner to which they're removably attached. They have very narrow slits the width of the thickness of the cutting blade edge that formed the edges of adjacent labels. So butt-cut labels make more efficient use of material, since there are no empty gaps between them in a roll, but they are more difficult to detect for automated processing unless registration marks are printed on either the labels themselves or the liner to which they're removably attached.
When labels are applied to products by machine they're most often detected by capacitive, ultrasonic, vision, or photoelectric-type sensors.
Capacitive sensors can have problems if there is any electrically conductive material in the liner or the labels. So their use can be problematic if the labels contain any metallic films or inks. Capacitive sensors are also problematic with butt-cut labels (regardless of their material content), because there is so little difference in the capacitance in the slits between the labels compared to the capacitance of the unslitted material of the labels themselves.
Ultrasonic sensors can work on die-cut labels of any material, but they also have problems detecting the edges of butt-cut labels, because they also have a hard time distinguishing the slits between the labels from the labels themselves.
Vision systems are generally used for label inspection rather than for simple detection of the location of labels due to their cost and complexity.
The remaining sensing technology commonly being used to detect label presence and/or location is photoelectric sensing. Photoelectric sensors are used in several different ways to detect labels' presence and/or position, depending on the type of photoelectric sensor that's chosen.
But the problem to-date with label detection using photoelectric sensors has been that detection is very dependent on the optical properties of the labels and the liner. And the optical characteristics of labels and liners can be all over the place: from clear to opaque, different colors, and reflective surface characteristics ranging from nearly mirror-like to diffuse. Achieving a design for a photoelectric sensor such that a single sensor can reliably detect die-cut labels made from all kinds of material and printing is difficult, and has been elusive so far.
And the detection of butt-cut labels by photoelectric or any other means continues to elude sensor designers unless registration marks are used, or the labels are actually partially removed from the liner such as described in U.S. Pat. No. 4,867,833 (McCoy, Sep. 19, 1989).
The McCoy patent discusses a method of detecting the position of butt-cut labels by passing the web over a curved surface. The curvature of the surface is chosen so the label edges become partially unstuck from the liner as the web is flexed when it moves and curls over the curvature. That accentuates the gap between the labels and enables the detection of the label edges with one of several off-the-shelf sensors.
The curvature required to do this, however, depends on the stiffness of the labels and the strength of bonding between the labels and the liner. So a device built according to McCoy could potentially work for some butt-cut labels and not for others: for example, if much thinner and more flexible label material was used, or if a more sticky adhesive were used. Also, once the label edges have become partially dislodged from the liner, it's possible they could become further dislodged to the point that they could stick to some object along their path, and cause a jam. This is a sensing tactic that may be useful when the labels are going to be immediately removed from the liner after sensing and applied to product, but may be problematic for other applications, such as counting the labels on a roll or checking for missing labels.
There is also the question of positional accuracy of label detection done this way. Today's capacitive label sensors claim positional accuracy of 0.4 mm (0.016 inch). The McCoy patent does not tell us how accurately the edge detection is once the edges of butt-cut labels are dislodged from the liner. But even if the label-edge detection is done repeatably for a specific label, it's possible that the positional accuracy will change from one specific label to another due to the differences in label stiffness and adhesive strength as mentioned earlier—possibly even for the same specific label when purchased from different suppliers or in different lots from the same supplier.
All other methods I've found to-date for detecting the position of butt-cut labels involve registration marks of some kind on the labels or liners. There has been a bit more of a variety of approaches to the problem of detecting die-cut labels, with clear die-cut labels receiving the most interest.
U.S. Pat. No. 4,685,982 (Kucheck, Aug. 11, 1987) describes a way to detect die-cut labels' position regardless of their opacity by detecting the gaps between them. It involves directing a light beam from above the side of the web down toward the labels' surfaces (forming an acute angle with the labels' surfaces). At about the point on the labels where the light beam hits the labels, there is an opaque bar that extends in the direction of the labels' motion. The bar is positioned such that it just barely clears the tops of the labels. So when a gap between labels moves through the light spot, the light passes under the bar and reflects off the surface of the liner, to be detected on the other side of the web. There is some discussion in Kucheck of the angle of incidence for the light beam, but nothing regarding its diameter.
But any method that relies on light reflecting off the surface of the liner does not work for detecting the position of butt-cut labels, because on the label-side of the web, there is none of the liner's surface exposed between labels. So the label-sensing method described in Kucheck does not work for butt-cut labels.
Neither of the above prior art patents made use of the edges of the labels in the process of detecting the labels. But U.S. Pat. No. 6,460,765 (Priesbsch, Oct. 8, 2002) does. This patent exploits the fact that, due to the way die-cut labels are made, the edges of the labels are not perpendicular to the top label surfaces. When viewed in cross-section, the edges have varying degrees of angularity. So the Priesbsch patent emits a small-diameter light beam down onto the passing labels perpendicularly from above, and uses the shape of the label edges to redirect some of the light beam into clear die-cut labels using refraction. Then the light reflects around inside the label, eventually exiting the top and bottom side of the label, where it impinges on one of the sensing receivers after having been displaced from its original path.
However, edge-refraction into the clear label material as described in Priesbsch only works for clear labels—not for labels made of opaque material. So the Priesbsch patent essentially reverts back to prior art technology of transmitting light through the labels and liner and then comparing light levels with threshold values to decide whether a label or gap has passed through the light beam when it's dealing with opaque labels.
U.S. Pat. No. 5,841,881 (Iwakawa et al, Nov. 24, 1998) uses edge-reflection to help detect the position of address labels and address windows on postal packages. The method uses is a vision system, but the illumination and optical design used to acquire the image data is of some interest here in terms of prior art. FIGS. 3A and 3B in Iwakawa show cross-sectional views of label edges, and rays illustrating the necessity for the light supplied by the light sources in Iwakawa to be converging as it approaches the label edges (FIGS. 3A and B in Iwakawa; column 4, lines 30-31; and claim 7).
But Iwakawa uses 2 light beams: one for preferentially illuminating the leading edges, and the other for preferentially illuminating the trailing edges, with the two beams pointing in opposite directions of each other (column 3, lines 50-56), and they are wide beams that illumate the entire width of the conveyor moving the postal packages (FIG. 1).
The two light beams in Iwakawa also do not directly detect any edges by means of their reflection: the edges are detected only after images of the passing postal packages are digitally processed by a computer (column 3, lines 34-44). The two light beams in Iwakawa are purely for illumination, that provides some preferential intensity variation when it reflects off the edges of the labels.
All the light that the two beams emit is used (one way or another) in the process of deciding where the label edges are located. The light that reflects off the labels' edges is neither redirected such that initially undetected light is detected, nor initially detected light goes undetected; but rather, to simply alter the intensity of light that forms the image that has to be processed by a computer to calculate where the edges are. So the only thing the edge-reflection of the light is doing in Iwakawa is a redistribution of the emitted light beam within the detected image—all the light reflected off the label edges is still detected, it's just moved from one place in the image to another place in the image. Since the light beams in Iwakawa are for illumination, and actual edge-detection only occurs after a computer analyzes the image, the light reflecting off ALL the surfaces and edges is collected and used when the image is analyzed by the computer. None of the light in Iwakawa has to reflect off a label edge in order to be detected, and none of the light in Iwakawa reflects off a label edge such that becomes undetected.
Iwakawa's light beam sources are located above the conveyor, pointing somewhat downward, but with one beam mostly in the direction of the conveyor's motion, and the other beam mostly opposite the direction of the conveyor's motion (FIGS. 1, 2A, 2B, 3A & 3B; and column 3, lines 45-56).
Iwakawa also states that the optical axis of the lens that focuses the light reflected off the label edges onto the image sensor is perpendicular to the plane formed by the conveyor, and hence the top surface of the labels (column 4, lines 4-6). So the image sensor is directly above the conveyor (FIG. 1).
U.S. Pat. No. 8,125,654 (Benvegnu et al, Feb. 28, 2012) uses edge-reflection of light to determine if the outside edge of a circular semiconductor wafer has been sufficiently polished. But the emphasis is more on spectrographic content of the reflected light than on the direction in which the light has reflected off the edge. And in one of the embodiments, the light beam is directed in a plane that intersects the center of the circular disk-shaped wafer (column 9, lines 35-52).
So there is very little in the prior art regarding the detection of butt-cut label edges, and few prior art examples of photoelectric sensors capable of detecting all kinds of die-cut labels. Today I know of no sensor short of a vision system that can detect both die-cut and butt-cut labels made of any material.